The present invention relates to photography and more particularly to the production of vesicular images in water-insensitive photographic materials.
Vesicular images are formed in a photographic film by small bubbles or vesicles of gas which are formed and trapped in the areas of the film exposed to light and which refract light. Generally speaking, the film has a colloid or a resin coating, referred to as a vehicle, on a backing material and a light-sensitive agent, most commonly a diazo compound, dispersed throughout the coating. When the film is selectively exposed to image-defining light, the light-sensitive agent is decomposed and releases molecules of a gas-nitrogen in the case of diazo compounds. The gas ordinarily does not form vesicles immediately, but does so when the film is developed by heating, presumably because the vehicle is relaxed sufficiently on heating for the gas molecules to form bubbles in the vehicle and for the bubbles to expand. The resulting vesicles make the vehicle opaque to transmission of light in the exposed areas and also reflect light and scatter light so that they appear white.
The early vesicular materials employed gelatin as the vehicle. These suffered from the difficulty that the vesicular images faded rapidly. Later work revealed that this problem was caused, in part, by the sensitivity of gelatin to water. Gelatin vehicles absorbed moisture from the atmosphere and became soft, thus collapsing the vesicles and destroying the image.
It is now preferred to employ polymers or resins as the vehicle. Vehicles which are particularly preferred include those described in Notley, U.S. Pat. No. 3,620,743 and Notley, U.S. Pat. No. 3,622,336. U.S. Pat. No. 3,620,743 discloses a vehicle made from a water insoluble polymer selected from a group consisting of homopolymers of .alpha.-chloroacrylonitrile and copolymers of .alpha.-chloroacrylonitrile with a different vinyl monomer in which the mole fraction of the vinyl monomer in the copolymer is less than 0.50. U.S. Pat. No. 3,622,336 discloses a vehicle which is a copolymer of .alpha.-chloroacrylonitrile and .alpha.-methacrylonitrile.
It has been shown previously by several investigators that poly(.alpha.-chloroacrylonitrile) degrades at high temperatures, e.g. 200.degree. C.-400.degree. C., to give colored conjugated linear and cyclicized polymers. A source for this observation is "Thermal Degradation of Poly(.alpha.-Chloroacrylonitrile)", N. Grassie and E. M. Grant, Journal of Polymer Science, No. 16, pp. 591-599 (1967). The mechanism for this degradation involves the loss of HCl at varying rates, and depending upon the temperature, the further loss of HCN. It has also been found that the loss of HCl can occur at much lower temperatures, i.e. 100.degree. C. The cause of loss of HCl at 100.degree. C. is believed to be attributable to the simultaneous presence of a diazonium salt and poly(.alpha.-chloroacrylonitrile) resin. Elimination of HCl can take place in a few days at 90.degree. C. and in several hours at 100.degree. C. This degradation has the undesirable effect of bringing about a purple, highly dense background to vesicular film, thus rendering the film unsuitable for imaging.